Multi-stage flash evaporator



United States Patent O ABSTRACT OF THE DISCLOSURE A multi-stage flashevaporator in which a succession of ilash chambers are interconnected byorices, and including means for supplying heated liquid to the rstchamber in the succession, means for condensing vapors produced in thesuccessive chambers, and for maintaining the chambers in the successionat successively decreasing vapor pressures. The arrangement is such thatthe vapor pressure differences between chambers at the beginning of thesuccession are larger than the vapor pressure differences betweenchambers later in the succession. There are at least two chambers at thebeginning of the succession of which the second, or in other words thechamber with the lower vapor pressure, is at a higher level than thefirst chamber, or in other words the chamber with the higher vaporpressure. Thus, the liquid tends, by reason of the vapor pressuredifferences between the chambers, to rise from one chamber to the next.

The invention relates to apparatus for the evaporation of liquids indistillation plants and more specifically relates to improvements inmulti-stage Hash evaporators.

In multi-stage flash evaporators a liquid to be evaporated incontinuously circulated from one stage (usually the last) through a tubeheat exchanger and a heater into another stage (usually the irst) of aseries of ilash chambers through which the liquid then iiows insuccession, a proportion of the liquid being evaporated in each chamberand the vapour being condensed by heat exchange with the liquid in theheat exchanger. A plurality of orifices connect the chambers togetherand, although a small fraction of the liquid is vapourised in eachchamber, the majority ow through the orifices between the stages whichare maintained as successively decreasing vapour pressures. The vapourpressure difference between stages at the high temperature end is, ormay be, greater than the pressure difference between stages at the lowtemperature end.

The invention provides a multi-stage liash evaporator in which there isa succession of flash chambers interconnected by orifices, means forsupplying heated liquid to the first chamber in the succession, meansfor condensing vapours produced in the successive chambers and formaintaining the chambers in the succession at successively decreasingvapour pressures, with the vapour pressure differences between chambersat the beginning of the succession larger than the vapour pressurediierences between chambers later in the succession, and in which thechambers at the beginning of the succession are at progressively higherlevels whereby the liquid is caused, by the vapour pressure diiferencesbetween the chambers, to rise from one chamber to the next.

Preferably the said chambers at the beginning of the succession arearranged in step-like form, the static head of liquid threby beinggradually and constantly increased.

In one form of the invention the chambers later in the succession arearranged in two (or more) tiers, one above the other, the circulation ofliquid progressing from the initial chambers through the top tier ofchambers and then through the lower tier of chambers.

In another form of the invention the chambers later in ice thesuccession are arranged at progressively lower levels. In this form thechambers may be arranged in step-like form, the static head of liquidthereby being gradually and constantly, decreased.

In yet another form, the chambers later in the succession are either allat the same height or the rst of these chambers are at the same heightand the remainder at progressively lower levels.

The circulation of liquid is preferably assisted by means of a pump.

The invention also provides the method of evaporating a liquid whichcomprises the steps of passing the liquid through a succession ofchambers, maintaining the vapour pressure in the chambers atsuccessively decreasing values along the succession, the dilierences inpressure between successive stages at the beginning of the successionbeing greater than the pressure differences between successive chamberslater in the succession, and utilising the greater pressure dilerencesto lift the liquid between stages at the beginning of the succession.

The flow of the liquid between chambers later in the succession may bedownward.

By way of example two forms of ash evaporator in accordance with theinvention will now be described with reference to the accompanyingdrawings in which:

FIGURE l is a diagrammatic view of a first form of evaporator, and

FIGURE 2 is a diagrammatic view of a second form of evaporator.

Referring to FIGURE l, the evaporator comprises a circuit through whicha liquid (e.g. brine) is continuously circulated. The circuit comprisesa heater 10, a control valve 9, a succession of heat evaporation stagesor chambers 11, a pump 12 and a tube system 13 for heat recovery. Brineis fed into the evaporator through a pipe 20 and is passed by the pump12 through the tube system 13 where it is heated and then to the heater10. The brine then ows into the chamber 14 at the high temperature endof the evaporator and thence through a series of orices 17, through thechambers to a chamber 15 at the low temperature end of the evaporator.The chamber 15 is cooled by externally supplied heat transfer means 16.As the vapor pressures of successive chambers are lower the liquidflashes as it enters a chamber and the flash distillate is condensed inthe tubes 13 and is collected in pans 21. The lblowdown is pumped fromthe apparatus at 22 by a pump 24. The distillate is pumped from theapparatus at 23 by a pump 25.

Erosion around the orifices 17 is dependent on the velocity of flowtherethrough. Hence, if the velocity of flow is reduced, the rate oferosion is also reduced. However, the velocity of flow through theorifices is dependent on the pressure di'erences on opposite sidesthereof. These differences are, in turn, dependent on the level of theliquid on the opposite sides and the pressure dilierences between pairsvof chambers, which diterences are greater at the high temperature endthan at the low temperature end of the succession.

It will be seen from FIGURE 1 that the initial chambers are arranged instep-like form one above the other to cause the liquid to rise from onechamber to the next. In this way the pressure difference across theorifice is reduced which 'not only reduces the velocity of flow andhence the rate of erosion of the orifices over this portion of theevaporator, but also greatly assists the action of the pump.

Referring to FIGURE 2, the circuit is identical to the circuit describedabove except for the arrangement of the chambers later in the successionwhich in this case are in step-like form one below the other. Thereferences of FIGURE 2 correspond to those of FIGURE l.

For further explanation of apparatus in accordance with the invention,the following table is included:

This table is for operation of the apparatus for water. Column 1 givesthe values of various characteristics of the ow of water across aninter-stage orifice when the general temperature is 250 F. (i.e. at thehigher temperature end of the evaporator) and the stage temperature dropper chamber is F. Column 2 gives the modied values when the pressuredifference between the stages is utilized to lift the water a givendistance between the stages. It will be noticed that the velocity of thewater through the orifices is considerably reduced. Column 3 gives thevalues when the general temperature is changed to 100 F. (i.e. at thelow temperature end of the evaporator and the stages are at the sameheight). Finally column 4 gives the modified values when the ow isassisted by the arrangement of the chamber as in FIGURE 2 where thepressure between stages is used to lift the water between stages. Itwill be seen that although the size of the orice required for thecircumstances of column 2 is greater than that required for thecircumstances of column l, this is more than off-set by the saving inorifice area at the low temperature end of the evaporator as shown incolumn 4 with regard to column 3.

I claim:

1. A multi-stage llash evaporator in which there is a succession ofilash chambers interconnected by orices, means for supplying heatedliquid for distillation to the first chamber in the succession, meanscomprising a condenser tube arranged in the upper section of saidchambers and through which said liquid ows for condensing vapoursproduced in the successive chambers and for maintaining the chambers inthe succession at successively decreasing vapour pressures, with thevapour pressure differences between chambers at the beginning of thesuccession larger than the vapour pressure differences between chamberslater in the succession, and in which said succession of chamberscomprise a rst group and a second group connected together, the firstgroup comprising a plurality of chambers arranged in step-wise form oneabove the other, the second group comprising a plurality of chambersarranged in step-wise form and in a downwardly inclined path one belowthe other to the last chamber.

2. A multi-stage evaporator as claimed in claim 1 and including a heatexchanger for condensing the vapours produced in the succession ofchambers and means for passing the liquid to be evaporated through theexchanger in heat exchange relation with the vapours and thence througha heater to the succession of chambers.

References Cited UNITED STATES PATENTS 1,782,959 ll/l930 Elliott202--173 2,979,443 4/1961 Frankel 202-173 3,l05,020 9/1963 Silver et al.202-173 X 3,119,752 l/l964 Checkovich 202-173 X 3,206,379 9/1965 Hill202-173 X 3,219,552 ll/l965 Starmer et al 202-173 X 3,219,553 ll/l965Hughes 202-173 OTHER REFERENCES Publication, Nat. Academy of Sciences,Proceedings of Symposium, Saline Water Conversion (1957) (pp. 92 and93).

NORMAN YUDKOFF, Primary Examiner.

F. E. DRUMMOND, Assistant Examiner.

1. A MULTI-STAGE FLASH EVAPORATOR IN WHICH THERE IS A SUCCESSION OFFLASH CHAMBERS INTERCONNECTED BY ORIFICES, MEANS FOR SUPPLYING HEATEDLIQUID FOR DISTILLATION TO THE FIRST CHAMBER IN THE SUCCESSION, MEANSCOMPRISING A CONDENSER TUBE ARRANGED IN THE UPPER SECTION OF SAIDCHAMBERS AND THROUGH WHICH SAID LIQUID FLOWS FOR CONDENSING VAPOURSPRODUCED IN THE SUCCESSIVE CHAMBERS AND FOR MAINTAINING THE CHAMBERS INTHE SUCCESSION AT SUCCESSIVELY DECREASING VAPOUR PRESSURE WITH THEVAPOUR PRESSURE DIFFERENCES BETWEEN CHAMBERS AT THE BEGINNING OF THESUCCESSION LARGER THAN THE VAPOUR PRESSURE DIFFERENCE BETWEEN CHAMBERSLATER IN THE SUCCESSION, AND IN WHICH SAID SUCCESSION OF CHAMBERSCOMPRISE A FIRST GROUP AND A SECOND